Tubular parts are commonly welded together by torch or arc welding processes such as TIG welding. For some wire-to-tube and tube-to-tube welding applications, such as in the manufacture of automobile seat-frame sections, it is important to ensure that the tubular parts are welded together properly. A seat-frame having defective torch or arc welded joints is expensive to replace once it is incorporated into a larger assembly. Inclusion of a defective seat-frame into a production vehicle, for example, can result in expensive warranty repairs or product recall.
The strength of a resistance welded joint depends upon the amount of penetration of the parts into each other during welding at the faying surfaces. Too much or too little weld penetration can result in a defective weld. Examples of systems monitoring the amount of weld penetration during welding are shown in U.S. Pat. No. 5,393,950 to Killian, U.S. Pat. No. 4,441,006 to Machida et al., and U.S. Pat. No. 4,296,304 to Defourney. Since the electrodes of a weld gun are in continuous contact with the parts during welding, each of these disclosed systems teaches the use of measuring electrode displacement to directly obtain the amount of weld penetration. However, these systems are directed to the resistance welding of planar parts and are not readily adaptable to tube welding applications.
Planar parts, unlike tubular parts, are usually not susceptible to undesired movement such as deformation caused by the force applied from the electrodes to the parts to effect resistance welding. Furthermore, planar parts are inherently stable and, unlike unstable tubular parts, they are also not as likely to roll, slide, or move with respect to one another when placed between the operative electrodes. As can be appreciated, any movement of the parts which is not caused by weld penetration will result in an erroneous reading of the amount of electrode displacement. This, in turn, will result in an erroneous calculation of the strength of the joint as can be appreciated when considering that electrode displacement is commonly measured with a resolution of 1/1000 of an inch.
Accordingly, it is an object of the present invention to provide an apparatus and method for properly resistance welding tubular parts.
It is another object of the present invention to provide an apparatus and method for obtaining a predetermined displacement of the electrodes during resistance welding to maximize the strength of the weld between the tubular parts.
Yet another object of the present invention is to provide an apparatus and method for determining whether the tubular parts are properly aligned prior to resistance welding.